Objective lens with center and periphery portions producing reciprocal interference, and optical head and optical disk device using the same

ABSTRACT

The objective lens of the present invention includes a center portion and a periphery portion surrounding the center portion. An aberration of the periphery portion is corrected such that a light spot is formed by the convergence of a luminous flux which has been transmitted through the periphery portion and then transmitted through a first light transmissive flat plate, and an aberration of the center portion is corrected such that a light spot is formed by the convergence of a luminous flux which has been transmitted through the center portion and then transmitted through a second light transmissive flat plate which is thicker than the first light transmissive flat plate.

This is a division of application Ser. No. 08/972,913, filed Nov. 18,1997, now U.S. Pat. No. 6,069,860.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an objective lens for converging thelight emitted from a light source onto the information recording surfaceof an optical disk, and also relates to an optical head and an opticaldisk device for optically recording/reproducing information onto/from anoptical disk using the objective lens.

2. Description of the Related Art

An objective lens used for an optical head is designed in view of thepredetermined base material thickness of an optical disk. Thus, when anoptical disk having a different base material thickness from thepredetermined thickness is installed, a spherical aberration is caused,so that the convergence performance is deteriorated and it becomesdifficult to precisely record/reproduce information onto/from theoptical disk. All of conventional optical disks, including a so-called“compact disk (CD)”, i.e., a read-only disk for music replay, a videodisk and a magneto-optical disk for data storage, have a uniform basematerial thickness of about 1.2 mm. Therefore, it has heretofore beenpossible to record/reproduce information from optical disks of varioustypes by using a single optical head.

On the other hand, a digital video disk (DVD), the specifications ofwhich have recently been unified, uses an objective lens having anincreased numerical aperture (NA) in order to realize a high density. Ifthe numerical aperture is increased, then the optical resolution of anoptical disk is improved. As a result, the width of a frequency band onwhich the recording/reproducing operations are enabled can be expanded.However, if an optical disk to be installed has a tilt, then a coma isadversely increased. In general, an optical disk is tilted to a certaindegree against an objective lens, because the optical disk itself has acertain deflection and some inclination is almost always involved whenthe optical disk is installed into an optical disk device. Consequently,a kind of aberration called “coma” is generated in a converged lightspot. The coma disadvantageously prevents the convergence performancefrom being improved even when the numerical aperture is increased.

Thus, the base material thickness of a DVD (it is noted that the “basematerial thickness” of a DVD corresponds to the thickness of one of apair of bonded substrates, unlike the cases of conventional opticaldisks) is reduced to about 0.6 mm such that the coma is not increasedeven when the numerical aperture of an objective lens is increased.However, reducing the base material thickness of an optical disk meanschanging the predetermined base material thickness for an objective lensfor recording/reproducing information onto/from the optical disk. As aresult, the objective lens for the changed base material thickness canno longer be used for recording/reproducing information onto/fromconventional optical disks (i.e., CDs, magneto-optical disks for datastorage, etc.), i.e., the objective lens is no longer compatible withthe conventional optical disks.

In order to solve such a problem, a device using two optical heads suchas that shown in FIG. 12 is proposed. The device shown in FIG. 12includes two optical heads 70 and 83. The optical head 70 is used forrecording/reproducing information onto/from an optical disk 10 having abase material thickness of about 0.6 mm, while the optical head 83 isused for recording/reproducing information onto/from an optical disk 11having a base material thickness of about 1.2 mm. It is noted that, inFIG. 12, the left half portion of the optical disk 10 having the basematerial thickness of about 0.6 mm and the right half portion of theoptical disk 11 having the base material thickness of about 1.2 mm areselectively illustrated.

In the optical head 70, the radiated light having a wavelength of about650 nm which has been emitted from a semiconductor laser device 71 iscondensed by a condenser lens 72 to be transformed into a luminous flux73 of substantially parallel light beams. The luminous flux 73 isp-polarized light, which is incident onto and transmitted through apolarization beam splitter 74 and transformed by a quarter-wave plate 75into substantially circularly polarized light. The circularly polarizedlight is reflected by a reflective mirror 76 to be incident onto anobjective lens 77. The luminous flux 73 transmitted through theobjective lens 77 is converged onto the information recording surface ofthe optical disk 10 having the base material thickness of about 0.6 mm,thereby forming a light spot 78 thereon.

The luminous flux reflected by the optical disk 10 passes through theobjective lens 77, the reflective mirror 76 and the quarter-wave plate75 again to be incident onto the polarization beam splitter 74. Sincethe reflected luminous flux is transformed by the quarter-wave plate 75into s-polarized light, the s-polarized light is reflected by thepolarization beam splitter 74, passed through a converging lens 79 and acylindrical lens 80, and is received by a photodetector 81. Thephotodetector 81 photoelectrically converts the received reflectedluminous flux to form a reproduced signal, forms a focusing controlsignal in accordance with an astigmatism method, forms a trackingcontrol signal in accordance with a phase difference method and apush-pull method, and then outputs these signals.

An objective lens driver 82 drives the objective lens 77 in the focusingdirection and the tracking direction, thereby making the light spot 78follow the tracks on the surface of the recording medium onto/from whichthe information is recorded/reproduced.

On the other hand, in the optical head 83, the radiated light having awavelength of about 780 nm which has been emitted from a semiconductorlaser device 84 is condensed by a condenser lens 85 to be transformedinto a luminous flux 86 of substantially parallel light beams. Theluminous flux 86 is p-polarized light, which is incident onto andtransmitted through a polarization beam splitter 87 and transformed by aquarter-wave plate 88 into substantially circularly polarized light. Thecircularly polarized light is reflected by a reflective mirror 89 to beincident onto an objective lens 90. The luminous flux 86 transmittedthrough the objective lens 90 is converged onto the informationrecording surface of the optical disk 11 having the base materialthickness of about 1.2 mm, thereby forming a light spot 91 thereon.

The luminous flux reflected by the optical disk 11 passes through theobjective lens 90, the reflective mirror 89 and the quarter-wave plate88 again to be incident onto the polarization beam splitter 87. Sincethe reflected luminous flux is transformed by the quarter-wave plate 88into s-polarized light, the s-polarized light is reflected by thepolarization beam splitter 87, passed through a converging lens 92 and acylindrical lens 93, and received by a photodetector 94. Thephotodetector 94 photoelectrically converts the received reflectedluminous flux to form a reproduced signal, forms a focusing controlsignal in accordance with an astigmatism method, forms a trackingcontrol signal in accordance with a phase difference method and apush-pull method, and then outputs these signals.

An objective lens driver 95 drives the objective lens 90 in the focusingdirection and the tracking direction, thereby making the light spot 91follow the tracks on the surface of the recording medium onto/from whichthe information is recorded/reproduced.

In the above-described arrangement, in the case of recording/reproducinginformation onto/from the optical disk 11 having a base materialthickness of about 1.2 mm such as a CD, the optical head 83 is operatedand controlled such that the light spot 91 is formed on the informationrecording surface of the optical disk 11. On the other hand, in the caseof recording/reproducing information onto/from the optical disk 10having a base material thickness of about 0.6 mm such as a DVD, theoptical head 70 is operated and controlled such that the light spot 78is formed on the information recording surface of the optical disk 10.In this way, information can be recorded/reproduced onto/from both theoptical disks 10 and 11 having different base material thicknesses.

However, in the above-described conventional arrangement, since twooptical heads are used, two optical systems including optical members,photodetectors, objective lens drivers, focusing drivers and trackingdrivers are required, so that the necessary costs are doubled.

Furthermore, though a distance S between the light spots 78 and 91 isconstant, the angle θ formed between the line linking the light spots 78and 91 and an information track 95 on the inner periphery of the opticaldisk is different from the angle θ formed between the line linking thelight spots 78 and 91 and an information track 95 on the outer peripheryof the optical disk. When the angles θ are variable, the diffractionpatterns of the information tracks 95 which are included in thereflected luminous flux rotate, thereby varying the levels of thetracking signals and deteriorating the quality of the tracking signals.

Moreover, when an optical disk is installed within a cartridge, the twoobjective lens drivers 82 and 95 are required to be disposed within anopening 96 of the cartridge. Thus, since the objective lens drivers 82and 95 must be downsized, the forces of the objective lens drivers 82and 95 for driving the objective lenses 77 and 90 are decreased, and itbecomes difficult to reproduce information from an optical disk byincreasing the rotations per minute of the optical disk.

SUMMARY OF THE INVENTION

The objective lens of the present invention includes a center portionand a periphery portion surrounding the center portion. An aberration ofthe periphery portion is corrected such that a light spot is formed bythe convergence of a luminous flux which has been transmitted throughthe periphery portion and then transmitted through a first lighttransmissive flat plate, and an aberration of the center portion iscorrected such that a light spot is formed by the convergence of aluminous flux which has been transmitted through the center portion andthen transmitted through a second light transmissive flat plate which isthicker than the first light transmissive flat plate.

The optical head of the present invention includes: a light source foremitting a luminous flux; an objective lens for converging the luminousflux onto either the information recording surface of a first lighttransmissive flat plate functioning as a recording medium or theinformation recording surface of a second light transmissive flat platealso functioning as a recording medium; and a photodetector fordetecting the luminous flux, which has been reflected by or transmittedthrough the information recording surface of the first or the secondlight transmissive flat plate, thereby outputting an electric signal.The objective lens includes a center portion and a periphery portionsurrounding the center portion. An aberration of the periphery portionis corrected such that a light spot is formed by the convergence of theluminous flux which has been transmitted through the periphery portionand then transmitted through the first light transmissive flat plate,and an aberration of the center portion is corrected such that a lightspot is formed by the convergence of the luminous flux which has beentransmitted through the center portion and then transmitted through thesecond light transmissive flat plate which is thicker than the firstlight transmissive flat plate.

In one embodiment, the first and the second light transmissive flatplates have a thickness t1 and a thickness t2, respectively, theaberration of the periphery portion is corrected such that a light spotis formed by the convergence of the luminous flux which has beentransmitted through the periphery portion of the objective lens and thentransmitted through the first light transmissive flat plate having thethickness t1, and the aberration of the center portion is corrected suchthat a light spot is formed by the convergence of the luminous fluxwhich has been transmitted through the center portion of the objectivelens and then transmitted through a light transmissive flat plate havinga thickness in the range from about (t2×0.7) to t2.

In another embodiment, the thickness t2 of the second light transmissiveflat plate is set at a thickness approximately twice as large as thethickness t1 of the first light transmissive flat plate.

In still another embodiment, the optical head further include aperturecontrol means for limiting the aperture of the objective lens. Theaperture of the objective lens is limited by the aperture control meanswhen the luminous flux emitted from the light source is converged ontothe information recording surface of the second light transmissive flatplate.

In still another embodiment, the optical head further includesseparation means for separating the luminous flux, which has beenreflected by or transmitted through either the information recordingsurface of the first light transmissive flat plate or the informationrecording surface of the second light transmissive flat plate, into afirst luminous flux which has been transmitted through the peripheryportion of the objective lens and a second luminous flux which has beentransmitted through the center portion of the objective lens. Thephotodetector detects the first and the second luminous fluxes, therebyoutputting respective electric signals. The electric signals of thephotodetector respectively corresponding to the first and the secondluminous fluxes are selected when the luminous flux emitted from thelight source is converged onto the information recording surface of thefirst light transmissive flat plate, and the electric signal of thephotodetector corresponding to the second luminous flux is selected whenthe luminous flux emitted from the light source is converged onto theinformation recording surface of the second light transmissive flatplate.

In still another embodiment, the separation means is a polarizinghologram.

In still another embodiment, assuming the wavelength of the luminousflux emitted from the light source is denoted by λ nm, the numericalaperture of the center portion of the objective lens is set to besubstantially equal to or smaller than (λ/780)×0.53.

In still another embodiment, assuming the wavelength of the luminousflux emitted from the light source is in the range from about 600 nm toabout 700 nm, the numerical aperture of the center portion of theobjective lens is set in the range from about 0.34 to about 0.4 and thenumerical aperture of the periphery portion of the objective lens is setto be substantially equal to about 0.6.

The optical head according to another aspect of the present inventionincludes: a first light source for emitting a first luminous flux; asecond light source for emitting a second luminous flux having awavelength different from a wavelength of the first luminous fluxemitted by the first light source; an objective lens for converging thefirst luminous flux onto the information recording surface of a firstlight transmissive flat plate functioning as a recording medium and forconverging the second luminous flux onto the information recordingsurface of a second light transmissive flat plate also functioning as arecording medium; and a photodetector for detecting the luminous flux,which has been reflected by or transmitted through the informationrecording surface of the first or the second light transmissive flatplate, thereby outputting an electric signal. The objective lensincludes a center portion and a periphery portion surrounding the centerportion. An aberration of the periphery portion is corrected such that alight spot is formed by the convergence of the first luminous flux whichhas been transmitted through the periphery portion and then transmittedthrough the first light transmissive flat plate, and an aberration ofthe center portion is corrected such that a light spot is formed by theconvergence of the second luminous flux which has been transmittedthrough the center portion and then transmitted through the second lighttransmissive flat plate which is thicker than the first lighttransmissive flat plate.

In one embodiment, assuming the thickness t1 of the first lighttransmissive flat plate to be about 0.6 mm and the thickness t2 of thesecond light transmissive flat plate to be about 1.2 mm, the aberrationof the periphery portion is corrected such that a light spot is formedby the convergence of the first luminous flux which has been transmittedthrough the periphery portion of the objective lens and then transmittedthrough the first light transmissive flat plate having the thickness ofabout 0.6 mm, and the aberration of the center portion is corrected suchthat a light spot is formed by the convergence of the second luminousflux which has been transmitted through the center portion of theobjective lens and then transmitted through a light transmissive flatplate having a thickness in the range from about 0.84 mm to about 1.2mm.

In another embodiment, the optical head further includes aperturecontrol means for limiting the aperture of the objective lens. Theaperture of the objective lens is limited by the aperture control meanswhen the second luminous flux emitted from the second light source isconverged onto the information recording surface of the second lighttransmissive flat plate.

In still another embodiment, the aperture control means is a wavelengthfilter for transmitting the second luminous flux emitted from the secondlight source and blocking the first luminous flux emitted from the firstlight source.

In still another embodiment, the optical head further includesseparation means for separating the luminous flux, which has beenreflected by or transmitted through either the information recordingsurface of the first light transmissive flat plate or the informationrecording surface of the second light transmissive flat plate, into thefirst luminous flux which has been transmitted through at least theperiphery portion of the objective lens and the second luminous fluxwhich has been transmitted through the center portion of the objectivelens. The photodetector detects the first and the second luminousfluxes, thereby outputting respective electric signals. The electricsignal of the photodetector corresponding to the first luminous flux isselected when the first luminous flux is converged onto the informationrecording surface of the first light transmissive flat plate, and theelectric signal of the photodetector corresponding to the secondluminous flux is selected when the second luminous flux is convergedonto the information recording surface of the second light transmissiveflat plate.

In still another embodiment, the separation means is a polarizinghologram.

In still another embodiment, assuming the wavelength of the firstluminous flux emitted from the first light source is in the range fromabout 600 nm to about 700 nm and the wavelength of the second luminousflux emitted from the second light source is in the range from about 750nm to about 860 nm, the numerical aperture of the center portion of theobjective lens is set to be substantially equal to about 0.45 and thenumerical aperture of the periphery portion of the objective lens is setto be substantially equal to about 0.6.

The optical head according to still another aspect of the presentinvention includes: a light source for emitting a luminous flux; anobjective lens for converging the luminous flux onto either theinformation recording surface of a first light transmissive flat platefunctioning as a recording medium or the information recording surfaceof a second light transmissive flat plate also functioning as arecording medium; separation means for separating the luminous flux,which has been reflected by or transmitted through either theinformation recording surface of the first light transmissive flat plateor the information recording surface of the second light transmissiveflat plate, into a first luminous flux which has been transmittedthrough the periphery portion of the objective lens and a secondluminous flux which has been transmitted through the center portion ofthe objective lens; and a photodetector for detecting the first and thesecond luminous fluxes, thereby outputting respective electric signals.

The optical disk device of the present invention, including an opticalhead, records/reproduces information onto/from the information recordingsurfaces of a first and a second optical disks which have respectivelydifferent thicknesses and are used as the first and the second lighttransmissive flat plates of the optical head. The optical head includesa light source for emitting a luminous flux; an objective lens forconverging the luminous flux onto either the information recordingsurface of a first light transmissive flat plate functioning as arecording medium or the information recording surface of a second lighttransmissive flat plate also functioning as a recording medium; and aphotodetector for detecting the luminous flux, which has been reflectedby or transmitted through the information recording surface of the firstor the second light transmissive flat plate, thereby outputting anelectric signal, the objective lens including a center portion and aperiphery portion surrounding the center portion, an aberration of theperiphery portion bring corrected such that a light spot is formed bythe convergence of the luminous flux which has been transmitted throughthe periphery portion and then transmitted through the first lighttransmissive flat plate, and an aberration of the center portion beingcorrected such that a light spot is formed by the convergence of theluminous flux which has been transmitted through the center portion andthen transmitted through the second light transmissive flat plate whichis thicker than the first light transmissive flat plate. The objectivelens of the optical head converges the luminous flux onto theinformation recording surfaces of the first and the second opticaldisks. The photodetector of the optical head detects the luminous flux,which has been reflected by or transmitted through the informationrecording surface of the first or the second disk, thereby outputting anelectric signal.

The optical disk device according another aspect of the presentinvention, including an optical head, records/reproduces informationonto/from the information recording surfaces of a first and a secondoptical disks which have respectively different thicknesses and are usedas the first and the second light transmissive flat plates of theoptical head. The optical head includes a first light source foremitting a first luminous flux; a second light source for emitting asecond luminous flux having a wavelength different from a wavelength ofthe first luminous flux emitted by the first light source; an objectivelens for converging the first luminous flux onto the informationrecording surface of a first light transmissive flat plate functioningas a recording medium and for converging the second luminous flux ontothe information recording surface of a second light transmissive flatplate also functioning as a recording medium; and a photodetector fordetecting the luminous flux, which has been reflected by or transmittedthrough the information recording surface of the first or the secondlight transmissive flat plate, thereby outputting an electric signal,the objective lens including a center portion and a periphery portionsurrounding the center portion, an aberration of the periphery portionbeing corrected such that a light spot is formed by the convergence ofthe first luminous flux which has been transmitted through the peripheryportion and then transmitted through the first light transmissive flatplate, and an aberration of the center portion being corrected such thata light spot is formed by the convergence of the second luminous fluxwhich has been transmitted through the center portion and thentransmitted through the second light transmissive flat plate which isthicker than the first light transmissive flat plate. The objective lensof the optical head converges the luminous flux onto the informationrecording surfaces of the first and the second optical disks. Thephotodetector of the optical head detects the luminous flux, which hasbeen reflected by or transmitted through the information recordingsurface of the first or the second disk, thereby outputting an electricsignal.

The optical disk device according still another aspect of the presentinvention, records/reproduces information onto/from the informationrecording surfaces of a first and a second optical disks which haverespectively different thicknesses and are used as the first and thesecond light transmissive flat plates of the optical head. The opticalhead includes: a light source for emitting a luminous flux; an objectivelens for converging the luminous flux onto either the informationrecording surface of a first light transmissive flat plate functioningas a recording medium or the information recording surface of a secondlight transmissive flat plate also functioning as a recording medium;separation means for separating the luminous flux, which has beenreflected by or transmitted through either the information recordingsurface of the first light transmissive flat plate or the informationrecording surface of the second light transmissive flat plate, into afirst luminous flux which has been transmitted through the peripheryportion of the objective lens and a second luminous flux which has beentransmitted through the center portion of the objective lens; and aphotodetector for detecting the first and the second luminous fluxes,thereby outputting respective electric signals. The objective lens ofthe optical head converges the luminous flux onto the informationrecording surfaces of the first and the second optical disks. Thephotodetector of the optical head detects the luminous flux, which hasbeen reflected by or transmitted through the information recordingsurface of the first or the second disk, thereby outputting an electricsignal.

Thus, the invention described herein makes possible the advantages of(1) providing an objective lens enabling a single optical head torecord/reproduce information onto/from optical disks having differentthicknesses, (2) providing an optical head using the objective lens, and(3) providing an optical disk device including the optical head.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the first example of the optical headof the present invention.

FIG. 2A shows the cross section and the upper surface of an objectivelens in the optical head shown in

FIG. 1, and FIG. 2B is a partially enlarged cross-sectional view of theobjective lens shown in FIG. 2A.

FIG. 3A is a view showing how information is recorded/reproduced by theoptical head shown in FIG. 1 onto/from an optical disk having a basematerial thickness of about 0.6 mm, and FIG. 3B is a view showing howinformation is recorded/reproduced by the optical head shown in FIG. 1onto/from an optical disk having a base material thickness of about 1.2mm.

FIG. 4A illustrates the function of the objective lens in the stateshown in FIG. 3A, and FIG. 4B illustrates the function of the objectivelens in the state shown in FIG. 3B.

FIG. 5 is a block diagram showing the second example of the optical headof the present invention.

FIG. 6 is a cross-sectional view of a polarizing hologram in the opticalhead shown in FIG. 5.

FIG. 7 is a partially enlarged view of the optical head shown in FIG. 5.

FIG. 8 is a block diagram showing the third example of the optical headof the present invention.

FIG. 9 shows the cross section and the upper surface of the objectivelens in the optical head shown in FIG. 8.

FIG. 10A is a view showing how information is recorded/reproduced by theoptical head shown in FIG. 8 onto/from an optical disk having a basematerial thickness of about 0.6 mm, and FIG. 10B is a view showing howinformation is recorded/reproduced by the optical head shown in FIG. 8onto/from an optical disk having a base material thickness of about 1.2mm.

FIG. 11 is a block diagram showing the fourth example of the opticalhead of the present invention.

FIG. 12 is a block diagram showing a conventional optical head.

FIG. 13 is a plan view illustrating the operation of the optical headshown in FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described by way ofillustrative examples with reference to the accompanying drawings.

EXAMPLE 1

FIG. 1 illustrates the first example of the optical head of the presentinvention.

In FIG. 1, the radiated light having a wavelength of about 650 nm whichhas been emitted from a semiconductor laser device 1 is condensed by acondenser lens 2 to be transformed into a luminous flux 3 ofsubstantially parallel light beams. The luminous flux 3 is p-polarizedlight, which is incident onto and transmitted through a polarizationbeam splitter 4.

The luminous flux 3 which has been transmitted through the polarizationbeam splitter 4 is transformed by a quarter-wave plate 5 from linearlypolarized light into substantially circularly polarized light, reflectedby a reflective mirror 6, passed through an aperture control section 7and then incident onto an objective lens 8.

As shown in FIG. 2A, the objective lens 8 includes a center portion A1,and a periphery portion A2 surrounding the center portion A1. Thenumerical aperture (NA) of the center portion A1 is about 0.37 and theNA of the periphery portion A2 is about 0.6. The center portion A1having the numerical aperture of about 0.37 is designed such that theaberration of a light spot to be transmitted through the center portionA1 and formed on the information recording surface of an optical diskbecomes minimum when the base material thickness of the optical disk isabout 0.9 mm. On the other hand, the periphery portion A2 having thenumerical aperture of about 0.6 is designed such that the aberration ofa light spot to be transmitted through the periphery portion A2 andformed on the information recording surface of an optical disk becomesminimum when the base material thickness of the optical disk is about0.6 mm.

FIG. 2B illustrates the cross-sectional shape of the upper portion ofthe objective lens 8. By forming the center portion A1 in the shapeshown in FIG. 2B, a light spot with a minimized aberration can be formedon the information recording surface of an optical disk having a basematerial thickness of about 0.9 mm. On the other hand, by forming theperiphery portion A2 in the shape shown in FIG. 2B, a light spot with aminimized aberration can be formed on the information recording surfaceof an optical disk having a base material thickness of about 0.6 mm. Thesurface region including the periphery portion A2 and the center portionA1 has a smoothly varying aspheric shape.

The aperture control section 7 is provided with a shutter 7 a which ismovable in a direction indicated by the arrow X in FIG. 1. When theshutter 7 a is moved to the left to be located on the optical path, theshutter 7 a limits the aperture of the objective lens 8. On the otherhand, when the shutter 7 a is moved to the right to be out of theoptical path, the shutter 7 a does not limit the aperture of theobjective lens 8.

FIG. 3A illustrates how information is recorded/reproduced onto/from theoptical disk 10 having a base material thickness of about 0.6 mm. Inthis case, the shutter 7 a is out of the optical path. Thus, thenumerical aperture of the objective lens 8 is not limited by the shutter7 a but is limited to about 0.6 by the inner diameter of an objectivelens holder 9. Consequently, the luminous flux 3 a is transmittedthrough the center portion A1 and the periphery portion A2 of theobjective lens 8 and converged by the objective lens 8 to form a lightspot 12 a on the information recording surface of the optical disk 10having a base material thickness of about 0.6 mm.

FIG. 3B illustrates how information is recorded/reproduced onto/from theoptical disk 11 having a base material thickness of about 1.2 mm, notfrom the optical disk 10. In this case, the shutter 7 a is moved ontothe optical path. Thus, the numerical aperture of the objective lens 8is limited by the shutter 7 a to about 0.37. Consequently, the luminousflux 3 b is transmitted only through the center portion A1 of theobjective lens 8 to form a light spot 12 b on the information recordingsurface of the optical disk 11 having a base material thickness of about1.2 mm.

The reflected luminous fluxes 13 a and 13 b which have been reflected bythe optical disks 10 and 11, respectively, are condensed again by theobjective lens 8, passed through the aperture control section 7, thereflective mirror 6 and the quarter-wave plate 5 and then incident ontothe beam splitter 4.

Since the reflected luminous fluxes 13 a and 13 b are transformed by thequarter-wave plate 5 into s-polarized light, the reflected luminousfluxes 13 a and 13 b are reflected by the beam splitter 4, passedthrough the converging lens 14 and the cylindrical lens 15 and receivedby the photodetector 16.

The photodetector 16 photoelectrically converts the received luminousfluxes 13 a and 13 b to form reproduced signals, forms focusing controlsignals in accordance with an astigmatism method, forms tracking controlsignals in accordance with a phase difference method and a push-pullmethod, and then outputs these signals.

An objective lens driver 17 drives the objective lens 8 in the focusingdirection and the tracking direction, thereby making the light spots 12a and 12 b respectively follow the tracks on the surface of the opticaldisks 10 and 11 onto/from which the information is recorded/reproduced.

Next, the objective lens 8 will be described in more detail below.

As described above, the numerical aperture of the periphery portion A2of the objective lens 8 is about 0.6. The periphery portion A2 isdesigned such that the aberration of the light spot 12 a formed on theinformation recording surface of the optical disk 10 having a basematerial thickness of about 0.6 mm becomes minimum when the luminousflux 3 a has been transmitted through the periphery portion A2 andconverged on the information recording surface.

When the luminous flux 3 a passes through the periphery portion A2, theluminous flux 3 a also passes through the center portion A1 of theobjective lens 8. Thus, as shown in FIG. 4A, the luminous fluxes whichhave respectively passed through the center portion A1 and the peripheryportion A2 are converged onto the information recording surface of theoptical disk 10 having a base material thickness of about 0.6 mm. Thecenter portion A1 is designed such that the aberration of the light spot12 a becomes minimum when the base material thickness of an optical diskis about 0.9 mm, as described above. In addition, the center portion A1and the periphery portion A2 are designed such that the first-order sidelobe of the light spot 12 a is minimized by the reciprocal interferenceof the respective luminous fluxes which have passed through the centerportion A1 and the periphery portion A2 to reach the informationrecording surface of the optical disk 10 having a base materialthickness of about 0.6 mm. As a result, the third-order sphericalaberration caused by the light spot 12 a converged by the center portionA1 and the periphery portion A2 can be suppressed to a similar level tothat of a conventional lens which has been exclusively designed so as toform a light spot with a minimized aberration on the informationrecording surface of an optical disk having a base material thickness ofabout 0.6 mm.

When the third-order spherical aberration is suppressed in this way, ahigher-order aberration is increased to the contrary. However, such ahigher-order aberration appears as a higher-order side lobe of the lightspot 12 a. Since the higher-order side lobe is distributed over a widerange, the reflected light caused by the higher-order side lobe does notcontain high-band signal components and the light amount thereof isaveraged. Thus, even when such reflected light is received andphotoelectrically converted by the photodetector 16, the signalcomponents thereof do not constitute noise components of the reproducedsignals, the focusing control signals and the tracking control signals,and therefore do not deteriorate these signals. Accordingly, even whenthe luminous flux 3 a is passed through the center portion A1 and theperiphery portion A2 so as to be converged, a sufficiently small lightspot 12 a can be formed, thereby precisely recording/reproducinginformation onto/from an optical disk having a base material thicknessof about 0.6 mm such as a DVD.

On the other hand, as also described above, the aberration of the lightspot formed on the information recording surface of an optical disk bymaking a luminous flux pass through the center portion A1 of theobjective lens 8 becomes minimum when the base material thickness of theoptical disk is about 0.9 mm. When the luminous flux 3 b which haspassed through the center portion A1 is converged on the informationrecording surface of the optical disk 11 having a base materialthickness of about 1.2 mm, the aberration of the light spot 12 b becomesapproximately half of the aberration defined by Marshall's criteria. Inother words, the size of the light spot 12 b can be reduced to apractical size.

As is clear from FIG. 4B, when the luminous flux passed through theperiphery portion A2 is converged on the information recording surfaceof the optical disk 11 having a base material thickness of about 1.2 mm,a blurred light spot is formed. On the other hand, when the luminousflux passed through the center portion A1 is converged on theinformation recording surface of the optical disk 11, a sufficientlysmall light spot 12 b is formed. Thus, if the numerical aperture of theobjective lens 8 is limited by the shutter 7 a to about 0.37 and theluminous flux 3 b is allowed to pass only through the center portion A1so as to be converged, then a sufficiently small light spot 12 b can beformed. Consequently, information can be precisely recorded/reproducedonto/from an optical disk having a base material thickness of about 1.2mm such as a CD.

If a luminous flux is converged onto an optical disk having a basematerial thickness of about 1.2 mm such as a CD by using a conventionalobjective lens designed exclusively for an optical disk having a basematerial thickness of about 0.6 mm such as a DVD, then a large sphericalaberration is caused so that the luminous flux is not converged into apoint. Though the spherical aberration is relatively small in theparaxial rays located in the vicinity of the optical axis, a criticalread error is likely to be caused when the base material thickness ofthe optical disk used is doubled. If an objective lens having anumerical aperture of about 0.6 which has been predetermined for anoptical disk having a base material thickness of about 0.6 mm is used,the numerical aperture of the objective lens is limited to about 0.37and a luminous flux is converged onto an optical disk having a basematerial thickness of about 1.2 mm, then a spherical aberration of about60 mλ is caused. If information is recorded/reproduced onto/from theoptical disk in such a state, then the resulting jitter is increased byabout 30% and the quality of the focusing signal is deteriorated. Thoughit is not impossible to reproduce information when the quality of thesignal is deteriorated to such a degree, it is desirable to reduce thespherical aberration in view of the variations of the environment,fabrication error of the optical disks and the like.

If the base material thickness of an optical disk which can form a lightspot with a minimized aberration through the center portion A1 of theobjective lens 8 having a numerical aperture of about 0.37 (hereinafter,such a base material thickness will be referred to as an “optimum basematerial thickness”) is set to be 70% of the base material thickness ofabout 1.2 mm of the optical disk (i.e., about 0.84 mm), then thespherical aberration can be reduced to about 40 mλ, and information canbe reproduced with substantially no error. Alternatively, the optimumbase material thickness of the center portion A1 having a numericalaperture of about 0.37 may be set in an approximate range from about0.84 mm to about 1.2 mm.

In the first example, since the optimum base material thickness of thecenter portion A1 of the objective lens 8 having a numerical aperture ofabout 0.37 is set at about 0.9 mm which is greater than about 0.6 mm,information can be precisely recorded/reproduced onto/from an opticaldisk having a base material thickness of about 1.2 mm.

A wavefront aberration is increased in an optical disk having a basematerial thickness of about 0.6 mm such as a DVD. However, since almostall of the wavefront aberration is a higher-order aberration, aspherical aberration, which is a critical problem during reproduction,can be suppressed to a low level.

Thus, the objective lens of the present invention hardly deterioratesthe performance of the optical head with respect to a DVD and canimprove the reproduction performance of the optical head with respect toa CD.

In a conventional device for reproducing information from a CD, thewavelength of a luminous flux emitted from a semiconductor laser deviceis set in the range from about 780 nm to about 820 nm and the numericalaperture of an objective lens is set at about 0.45. On the other hand,in various information recording/reproducing devices for a video diskand the like, the numerical aperture of an objective lens is furtherincreased to about 0.53.

In this first example, since the wavelength of the luminous flux emittedfrom the semiconductor laser device 1 is set at about 650 nm, thenumerical aperture of the center portion A1 of the objective lens 8 isset in accordance with the wavelength of about 650 nm. If a device inwhich the base material thickness of an optical disk is set at about 1.2mm, the wavelength of the luminous flux emitted from a semiconductorlaser device is set at about 780 nm, and the numerical aperture of anobjective lens is set at about 0.45 is assumed to be used, then thenumerical aperture required for realizing a similar performance to thatof such a device by setting the wavelength λ at an appropriate value isobtained by (λ/780)×0.45. Since the wavelength of the luminous flux isabout 650 nm in this case, the numerical aperture becomes(650/780)×0.45=0.375, which is approximately equal to the numericalaperture (=about 0.37) of the center portion A1 of the objective lens 8.

In a conventional device for reproducing information from a DVD, thewavelength of a luminous flux emitted from a semiconductor laser deviceis set in the range from about 635 nm to about 660 nm and the numericalaperture of the objective lens is set at about 0.6 in many cases. Inaddition, the wavelength of the luminous flux is possibly set in therange from about 600 nm to about 700 nm.

In the first example, the wavelength of a luminous flux emitted from thesemiconductor laser device 1 is set at about 650 nm and the numericalaperture of the periphery portion A2 of the objective lens 8 is set atabout 0.6. Thus, the optical disk device of this example can maintain areproduction ability comparable to that of a conventional device.

EXAMPLE 2

FIG. 5 illustrates the second example of the optical head of the presentinvention.

In FIG. 5, the radiated light having a wavelength of about 650 nm whichhas been emitted from a semiconductor laser device 21 is condensed by acondenser lens 22 to be transformed into a luminous flux 23 ofsubstantially parallel light beams. The luminous flux 23 is p-polarizedlight, which is incident onto and transmitted through a polarizationbeam splitter 24. The luminous flux 23 which has been transmittedthrough the polarization beam splitter 24 is reflected by a reflectivemirror 25 and is incident onto a polarizing hologram 26.

As shown in FIG. 6, the polarizing hologram 26 is provided with ahologram on a LiNb substrate having birefringence properties and isconfigured so as to diffract ordinary light and to transmitextraordinary light. Alternatively, the hologram may be formed bysubjecting the LiNb substrate to a proton exchange.

Almost all the components of the luminous flux 23 are those ofextraordinary light, which is incident onto and transmitted through thepolarizing hologram 26. A quarter-wave plate 27 is integrated with thepolarizing hologram 26. The luminous flux 23 passes through thequarter-wave plate 27 for transforming linearly polarized light intocircularly polarized light, and is incident onto an objective lens 28.An objective lens holder 29 limits the aperture of the objective lens 28such that the numerical aperture thereof becomes about 0.6.

The objective lens 28 includes a center portion A1 and periphery portionA2 surrounding the center portion A1, in the same way as the objectivelens 8 shown in FIG. 2A. The numerical aperture (NA) of the centerportion A1 is about 0.37 and the NA of the periphery portion A2 is about0.6. The center portion A1 having the numerical aperture of about 0.37is designed such that the aberration of a light spot passed through thecenter portion A1 and formed on the information recording surface of anoptical disk becomes minimum when the base material thickness of theoptical disk is about 0.9 mm. On the other hand, the periphery portionA2 having the numerical aperture of about 0.6 is designed such that theaberration of a light spot passed through the periphery portion A2 andformed on the information recording surface of an optical disk becomesminimum when the base material thickness of the optical disk is about0.6 mm.

The luminous flux 23 converged by the objective lens 28 forms a lightspot 12 a on the information recording surface of the optical disk 10having a base material thickness of about 0.6 mm or forms a light spot12 b on the information recording surface of the optical disk 11 havinga base material thickness of about 1.2 mm.

Next, the reflected luminous flux 31 reflected by the optical disk 10 or11 is condensed again by the objective lens 28, transformed by thequarter-wave plate 27 from substantially circularly polarized light intolinearly polarized light which is orthogonal to the luminous flux 23,and incident onto the polarizing hologram 26. Thus, the reflected light31 is incident as ordinary light onto the polarizing hologram 26 andthen diffracted by the polarizing hologram 26 to be divided into threereflected luminous fluxes 32, 33 and 34, as shown in FIG. 7.

The polarizing hologram 26 includes a center region and a peripheryregion corresponding to the center portion A1 and the periphery portionA2 of the objective lens 28, respectively, and the patterns of thehologram are designed so as to correspond to the respective regions.Consequently, the luminous flux which has passed through the centerportion A1 having a numerical aperture of about 0.37 and then passedthrough the center region of the polarizing hologram 26 becomes thereflected luminous flux 32, and the luminous fluxes which have passedthrough the periphery portion A2 having a numerical aperture in therange from about 0.37 to about 0.6 and then passed through the peripheryregion of the polarizing hologram 26 become the reflected luminousfluxes 33 and 34.

The reflected luminous fluxes 32, 33 and 34 are reflected by thereflective mirror 25 and incident onto the polarization beam splitter24. Since these reflected luminous fluxes 32, 33 and 34 are transformedby the quarter-wave plate 27 into s-polarized light and then incidentonto the polarization beam splitter 24, these luminous fluxes arereflected by the polarization beam splitter 24, passed through aconverging lens 35 and a cylindrical lens 36 and then received byphotodetectors 37, 38 and 39, respectively.

The photodetector 37 receives the reflected luminous flux 32 to form areproduced signal, forms a focusing control signal in accordance with anastigmatism method, forms a tracking control signal in accordance with aphase difference method and a push-pull method, and then outputs thesesignals. Similarly, the two other photodetectors 38 and 39 receive theluminous fluxes 33 and 34, respectively, and form and output therespective reproduced signals.

An objective lens driver 40 drives the objective lens 28 in the focusingdirection and the tracking direction, thereby making the light spots 12a and 12 b follow the tracks on the surface of the optical disks 10 and11 onto/from which the information is recorded/reproduced.

Alternatively, the polarizing hologram 26 and the quarter-wave plate 27may be integrated with the objective lens 28 and the assembly may bedriven by the objective lens driver 40.

In the above-described arrangement, in order to set the numericalaperture of the objective lens 28 at about 0.6 so that information isreproduced from the optical disk 10 having a base material thickness ofabout 0.6 mm, it is necessary to select all the luminous fluxes whichhave been passed through the center portion A1 and the periphery portionA2 of the objective lens 28, i.e., all of the reflected luminous fluxes32, 33 and 34. For such a purpose, a sum of the signals reproduced bythe respective photodetectors 37, 38 and 39 is obtained, and the sumsignal is used. In this case, information is reproduced from the opticaldisk 10 by using all the reflected luminous flux 31, i.e., all of theluminous fluxes 32, 33 and 34.

On the other hand, in order to set the numerical aperture of theobjective lens 28 at about 0.37 so that information is reproduced fromthe optical disk 11 having a base material thickness of about 1.2 mm, itis necessary to select a part of the luminous flux 31 which have beenpassed through the center portion A1 of the objective lens 28, i.e.,only the reflected luminous flux 32. For such a purpose, only the signalreproduced by the photodetector 37 is used.

In this way, the reflected luminous flux 32 which has passed through thecenter portion A1 having a numerical aperture of about 0.37 and theluminous fluxes 33 and 34 which have passed through the peripheryportion A2 having a numerical aperture in the range from about 0.37 toabout 0.6 are formed by making the polarizing hologram 26 separate thereflected luminous flux 31 reflected by the optical disk 10 or 11,instead of substantially limiting the aperture of the objective lens 28.These reflected luminous fluxes 32, 33 and 34 are individually detectedby the photodetectors 37, 38 and 39, respectively, and then thereproduced signals of the photodetectors 37, 38 and 39 are selectivelyused. In such a case, since no mechanical drive system is required forlimiting the aperture of the objective lens, a downsized and highlyreliable optical head can be formed.

EXAMPLE 3

FIG. 8 illustrates the third example of the optical head of the presentinvention.

In FIG. 8, radiated light having a wavelength of about 650 nm which hasbeen emitted from a first semiconductor laser device 41 is p-polarizedlight, which is transmitted through a first polarization beam splitter42, and then incident onto an optical path synthesizer/separator 43. Theoptical path synthesizer/separator 43 is configured so as to transmit aluminous flux having a wavelength of about 650 nm and to reflect aluminous flux having a wavelength of about 780 nm. Thus, the radiatedlight having a wavelength of about 650 nm which has been emitted fromthe first semiconductor laser device 41 is transmitted through theoptical path synthesizer/separator 43 and then condensed by a condenserlens 46 to be transformed into a luminous flux 47 of substantiallyparallel light beams.

The luminous flux 47 passes through a quarter-wave plate 49 fortransforming linearly polarized light into substantially circularlypolarized light, a reflective mirror 50 and a wavelength filter 51 so asto be incident onto an objective lens 52.

The wavelength filter 51 is configured such that the region thereofcorresponding to the center portion of the objective lens 52 having anumerical aperture of about 0.45 or less transmits light having awavelength of about 650 nm and light having a wavelength of about 780nm, and that the region thereof corresponding to the periphery portionof the objective lens 52 having a numerical aperture larger than about0.45 transmits light having a wavelength of about 650 nm but reflectslight having a wavelength of about 780 nm.

Thus, the luminous flux 47 having a wavelength of about 650 nm istransmitted through the wavelength filter 51. In this case, thenumerical aperture of the objective lens 52 is limited by an objectivelens holder 53 to about 0.6. The luminous flux 47 transmitted throughthe objective lens 52 with the numerical aperture limited to about 0.6is converged by the objective lens 52 to form a light spot 54 on theinformation recording surface of the optical disk 10 having a basematerial thickness of about 0.6 mm.

On the other hand, radiated light having a wavelength of about 780 nmwhich has been emitted from a second semiconductor laser device 44 isalso p-polarized light, which is transmitted through a secondpolarization beam splitter 45, and then incident onto the optical pathsynthesizer/separator 43. The optical path synthesizer/separator 43 isconfigured so as to reflect a luminous flux having a wavelength of about780 nm. Thus, the radiated light having a wavelength of about 780 nm isreflected by the optical path synthesizer/separator 43 and thencondensed by the condenser lens 46 to be transformed into a luminousflux 48 of substantially parallel light beams.

The luminous flux 48 passes through the quarter-wave plate 49 fortransforming linearly polarized light into substantially circularlypolarized light, the reflective mirror 50 and the wavelength filter 51so as to be incident onto the objective lens 52. Since the luminous flux48 has a wavelength of about 780 nm, the luminous flux 48 is reflectedby the region of the wavelength filter 51 corresponding to the peripheryportion of the objective lens 52 having a numerical aperture larger thanabout 0.45 and is transmitted by the region of the wavelength filter 51corresponding to the center portion of the objective lens 52 having anumerical aperture of about 0.45 or less. As a result, the numericalaperture of the objective lens 45 is substantially limited to about0.45. When the numerical aperture is limited to about 0.45, the luminousflux 48 is converged by the objective lens 52 to form a light spot 55 onthe information recording surface of the optical disk 11 having a basematerial thickness of about 1.2 mm.

The objective lens 52 is designed as shown in FIG. 9. Specifically, thecenter portion B1 having the numerical aperture of about 0.45 isdesigned such that the aberration of a light spot formed on theinformation recording surface of an optical disk becomes minimum whenthe base material thickness of the optical disk is about 0.9 mm. On theother hand, the periphery portion B2 having the numerical aperture inthe range from about 0.45 to about 0.6 is designed such that theaberration of a light spot formed on the information recording surfaceof an optical disk becomes minimum when the base material thickness ofthe optical disk is about 0.6 mm. The surface of the objective lens 52through which the luminous fluxes are transmitted is a continuous andsmooth spherical surface.

FIG. 10A illustrates how information is recorded/reproduced onto/fromthe optical disk 10 having a base material thickness of about 0.6 mm. Inthis case, the luminous flux 47 having a wavelength of about 650 nmwhich has been emitted from the first semiconductor laser device 41 istransmitted through both the center portion B1 and the periphery portionB2 of the objective lens 52. Thus, the numerical aperture of theobjective lens 52 is limited to about 0.6. The reflected luminous flux56 reflected by the optical disk 10 is condensed again by the objectivelens 52 and passed through the wavelength filter 51, the reflectivemirror 50, the quarter-wave plate 49 and the optical pathsynthesizer/separator 43 so as to be incident onto the firstpolarization beam splitter 42. The reflected luminous flux 56 istransformed by the quarter-wave plate 49 into s-polarized light, whichis reflected by the first polarization beam splitter 42, passed througha first cylindrical lens 58 and is received by a first photodetector 59.The first photodetector 59 photoelectrically converts the receivedreflected luminous flux 56 to form a reproduced signal, forms a focusingcontrol signal in accordance with an astigmatism method, forms atracking control signal in accordance with a phase difference method anda push-pull method, and then outputs these signals.

FIG. 10B illustrates how information is recorded/reproduced onto/fromthe optical disk 11 having a base material thickness of about 1.2 mm. Inthis case, the luminous flux 48 having a wavelength of about 780 nmwhich has been emitted from the second semiconductor laser device 44 istransmitted only through the center portion B1 of the objective lens 52.Thus, the numerical aperture of the objective lens 52 is limited toabout 0.45. The reflected luminous flux 57 reflected by the optical disk11 is condensed again by the objective lens 52, passed through thewavelength filter 51, the reflective mirror 50 and the quarter-waveplate 49 and reflected by the optical path synthesizer/separator 43 soas to be incident onto the second polarization beam splitter 45. Thereflected luminous flux 57 is transformed by the quarter-wave plate 49into s-polarized light, which is reflected by the second polarizationbeam splitter 45, passed through a second cylindrical lens 60 and isreceived by a second photodetector 61. The second photodetector 61photoelectrically converts the received reflected luminous flux 57 toform a reproduced signal, forms a focusing control signal in accordancewith an astigmatism method, forms a tracking control signal inaccordance with a phase difference method and a push-pull method, andthen outputs these signals.

In this way, the aberration in the center portion B1 having a numericalaperture of about 0.45 is corrected such that the optimum base materialthickness of the optical disk becomes about 0.9 mm. When information isreproduced from the optical disk 11 having a base material thickness ofabout 1.2 mm such as a CD, the numerical aperture of the objective lens52 is limited to about 0.45. As a result, the aberration of the lightspot can be set at a level comparable with a conventional objectivelens, the spherical aberration of which has been wholly corrected suchthat the optimum base material thickness becomes about 0.6 mm.Furthermore, when information is reproduced from the optical disk 10having a base material thickness of about 0.6 mm such as a DVD, thethird-order spherical aberration of the objective lens is not increasedeven if the numerical aperture of the objective lens 52 is set at about0.6.

In this third example, the aberration of the center portion B1 of theobjective lens 52 is corrected such that the optimum base materialthickness becomes about 0.9 mm. Alternatively, the aberration of thecenter portion B1 may be corrected such that the optimum base materialthickness becomes 70% or more of the base material thickness of theoptical disk 11, in the same way as in the first example illustrated inFIG. 1.

Moreover, in this example, the wavelength filter 51 is used as a meansfor limiting the aperture of the luminous flux 48. However, the sameeffects can also be attained even when the aperture of the reflectedluminous flux 57 is limited by using a polarizing hologram, as describedin the second example illustrated in FIGS. 4A and 4B.

EXAMPLE 4

FIG. 11 illustrates the fourth example of the optical head of thepresent invention.

In FIG. 11, the radiated light having a wavelength of about 650 nm whichhas been emitted from a semiconductor laser device 1 is condensed by acondenser lens 2 to be transformed into a luminous flux 3 ofsubstantially parallel light beams. The luminous flux 3 is p-polarizedlight, which is incident onto and transmitted through a polarizationbeam splitter 4. The luminous flux 3 which has been transmitted throughthe polarization beam splitter 4 is passed through and transformed by aquarter-wave plate 5 from linearly polarized light into substantiallycircularly polarized light, reflected by a reflective mirror 6, and thenincident onto an objective lens 8. An objective lens holder 9 limits thenumerical aperture of the objective lens 8 to about 0.6.

As described with reference to FIG. 2A, the numerical aperture (NA) ofthe center portion A1 of the objective lens 8 is about 0.37 and the NAof the periphery portion A2 thereof is about 0.6. The center portion A1having the numerical aperture of about 0.37 is designed such that theaberration of a light spot formed on the information recording surfaceof an optical disk becomes minimum when the base material thickness ofthe optical disk is about 0.9 mm. On the other hand, the peripheryportion A2 having the numerical aperture of about 0.6 is designed suchthat the aberration of a light spot formed on the information recordingsurface of an optical disk becomes minimum when the base materialthickness of the optical disk is about 0.6 mm.

The luminous flux 3 converged by the objective lens 8 forms a light spot12 a on the information recording surface of the optical disk 10 havinga base material thickness of about 0.6 mm or forms a light spot 12 b onthe information recording surface of the optical disk 11 having a basematerial thickness of about 1.2 mm.

Next, the reflected luminous flux 61 reflected by the optical disk 10 or11 is condensed again by the objective lens 8 and transformed by thequarter-wave plate 5 from substantially circularly polarized light intolinearly polarized light which is orthogonal to the luminous flux 3.Thus, the reflected light 61 is incident as s-polarized light onto andreflected by the polarization beam splitter 4 and passed through aconverging lens 62 to be incident onto a light separation mirror 63.

The light separation mirror 63 transmits the luminous flux transmittedthrough the center portion A1 of the objective lens 8 corresponding to anumerical aperture of about 0.37 and reflects the luminous fluxtransmitted through the periphery portion A2 of the objective lens 8.The reflected luminous flux 64 transmitted through the light separationmirror 63 has an astigmatism because the luminous flux 64 has beentransmitted through the light separation mirror 63 having aninclination. The luminous flux 64 is received by a first photodetector66. On the other hand, the reflected luminous flux 65 reflected by thelight separation mirror 63 is received by a second photodetector 67.

The first photodetector 66 photoelectrically converts the receivedluminous flux 64 to form a reproduced signal, forms a focusing controlsignal in accordance with an astigmatism method, forms a trackingcontrol signal in accordance with a phase difference method and apush-pull method, and then outputs these signals. On the other hand, thesecond photodetector 67 receives the reflected luminous flux 65 andforms and outputs a reproduced signal.

An objective lens driver 17 drives the objective lens 8 in the focusingdirection and the tracking direction, thereby making the light spot 12 aor 12 b follow the tracks on the surface of the optical disk 10 or 11onto/from which the information is recorded/reproduced.

In the above-described arrangement, in order to set the numericalaperture of the objective lens 8 at about 0.6 so that information isreproduced from the optical disk 10 having a base material thickness ofabout 0.6 mm, it is necessary to select all of the luminous flux whichhas been passed through the center portion A1 and the periphery portionA2 of the objective lens 8, i.e., the reflected luminous fluxes 64 and65. For such a purpose, a sum of the signals reproduced by the first andthe second photodetectors 66 and 67 is obtained, and the sum signal isused. On the other hand, in order to set the numerical aperture of theobjective lens 8 at about 0.37 so that information is reproduced fromthe optical disk 11 having a base material thickness of about 1.2 mm, itis necessary to select a part of the luminous flux 61 which has beentransmitted through the center portion A1 of the objective lens 8, i.e.,only the reflected luminous flux 64. For such a purpose, only the signalreproduced by the first photodetector 66 is used.

In this way, instead of substantially limiting the aperture of theobjective lens 8, the reflected luminous flux 61 which has beenreflected by the optical disk 10 or 11 is separated by the lightseparation mirror 63, thereby forming the reflected luminous flux 64which has passed through the center portion A1 having a numericalaperture of about 0.37 and the luminous flux 65 which has passed throughthe periphery portion A2 corresponding to a numerical aperture in therange from about 0.37 to about 0.6. These reflected luminous fluxes 64and 65 are individually detected by the first and the secondphotodetectors 66 and 67 and then the reproduced signals of the firstand the second photodetectors 66 and 67 are selectively used. In such acase, since no mechanical drive system is required for limiting theaperture of the objective lens, a downsized and highly reliable opticalhead can be formed.

In the foregoing examples, the present invention has been described asbeing applied to an objective lens and an optical head. However, itshould be noted that an optical disk device (i.e., a device forrecording/reproducing information onto/from a CD, a DVD and the like)including the objective lens and the optical head of the presentinvention also falls within the scope of the present invention.

As is apparent from the foregoing description, instead of providing anobjective lens for a DVD which is optimized for an optical disk having abase material thickness of about 0.6 mm, the present invention providesan objective lens, the aberration of the center portion of which iscorrected so as to form a light spot with a minimized aberration on anoptical disk having a base material thickness in the range from about0.84 mm to about 1.2 mm and the aberration of the periphery portion ofwhich is corrected so as to form a light spot with a minimizedaberration on an optical disk having a base material thickness of about0.6 mm, thereby recording/reproducing information onto/from both a CDhaving a base material thickness of about 1.2 mm and a DVD having a basematerial thickness of about 0.6 mm.

In addition, in the case of recording/reproducing information onto/froma CD, the aperture of an objective lens is limited to the center portionthereof, thereby forming a light spot of the same size as that of alight spot formed by a conventional optical head exclusively used for aCD. Consequently, an optical head for a DVD can be provided with reducedcosts and without using two optical systems and two objective lensdrivers (or focusing drivers and tracking drivers) which are required inthe conventional example shown in FIG. 12.

Furthermore, since the present invention can downsize an optical headwithout deteriorating the quality of a tracking signal as compared witha conventional optical head, information can be reproduced easily froman optical disk installed within a cartridge.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. An objective lens comprising a center portion and a periphery portion surrounding the center portion, wherein the periphery portion is formed such that an aberration of a light spot formed by the convergence of a luminous flux which has been transmitted through the periphery portion and a first light transmissive flat plate becomes minimum, the center portion is formed such that an aberration of a light spot formed by convergence of a luminous flux which has been transmitted through the center portion and a second light transmissive flat plate, which is thicker than the first light transmissive flat plate, becomes minimum, and the periphery portion and the center portion are designed so that a reciprocal interference of the respective luminous fluxes which have passed through the periphery portion and the center portion is produced.
 2. An optical disk device, comprising an optical head, for recording/reproducing information onto/from the information recording surfaces of a first and a second optical disks which have respectively different thicknesses and are used as the first and the second light transmissive flat plates of the optical head, the optical head including: a light source for emitting a luminous flux; an objective lens for converging the luminous flux onto either the information recording surface of a first light transmissive flat plate functioning as a recording medium or the information recording surface of a second light transmissive flat plate also functioning as a recording medium; separation means for separating the luminous flux, which has been reflected by or transmitted through either the information recording surface of the first light transmissive flat plate or the information recording surface of the second light transmissive flat plate, into a first luminous flux which has been transmitted through the periphery portion of the objective lens and a second luminous flux which has been transmitted through the center portion of the objective lens; and a photodetector for detecting the first and the second luminous fluxes, thereby outputting respective electric signals, wherein the objective lens of the optical head converges the luminous flux onto the information recording surfaces of the first and the second optical disks, the periphery portion and the center portion of the objective lens being designed so that a reciprocal interference of the respective luminous fluxes which have passed through the periphery portion and the center portion is produced, and the photodetector of the optical head detects the luminous flux, which has been reflected by or transmitted through the information recording surface of the first or the second disk, thereby outputting an electric signal.
 3. An optical disk device, comprising an optical head, for recording/reproducing information onto/from the information recording surfaces of a first and a second optical disks which have respectively different thicknesses and are used as the first and the second light transmissive flat plates of the optical head, the optical head including a light source for emitting a luminous flux; an objective lens for converging the luminous flux onto either the information recording surface of a first light transmissive flat plate functioning as a recording medium or the information recording surface of a second light transmissive flat plate also functioning as a recording medium; and a photodetector for detecting the luminous flux, which has been reflected by or transmitted through the information recording surface of the first or the second light transmissive flat plate, thereby outputting an electric signal, the objective lens including a center portion and a periphery portion surrounding the center portion, an aberration of the periphery portion being corrected such that a light spot is formed by the convergence of the luminous flux which has been transmitted through the periphery portion and then transmitted through the first light transmissive flat plate, and an aberration of the center portion being corrected such that a light spot is formed by the convergence of the luminous flux which has been transmitted through the center portion and then transmitted through the second light transmissive flat plate which is thicker than the first light transmissive flat plate, wherein the objective lens of the optical head converges the luminous flux onto the information and recording surfaces of the first and the second optical disks, the periphery portion and the center portion of the objective lens being designed so that a reciprocal interference of the respective luminous fluxes which have passed through the periphery portion and the center portion is produced, and the photodetector of the optical head detects the luminous flux, which has been reflected by or transmitted through the information recording surface of the first or the second disk, thereby outputting an electric signal.
 4. An optical disk device, comprising an optical head, for recording/reproducing information onto/from the information recording surfaces of a first and a second optical disks which have respectively different thicknesses and are used as the first and the second light transmissive flat plates of the optical head, the optical head including a first light source for emitting a first luminous flux; a second light source for emitting a second luminous flux having a wavelength different from a wavelength of the first luminous flux emitted by the first light source; an objective lens for converging the first luminous flux onto the information recording surface of a first light transmissive flat plate functioning as a recording medium and for converging the second luminous flux onto the information recording surface of a second light transmissive flat plate also functioning as a recording medium; and a photodetector for detecting the luminous flux, which has been reflected by or transmitted through the information recording surface of the first or the second light transmissive flat plate, thereby outputting an electric signal, the objective lens including a center portion and a periphery portion surrounding the center portion, an aberration of the periphery portion being corrected such that a light spot is formed by the convergence of the first luminous flux which has been transmitted through the periphery portion and then transmitted through the first light transmissive flat plate, and an aberration of the center portion being corrected such that a light spot is formed by the convergence of the second luminous flux which has been transmitted through the center portion and then transmitted through the second light transmissive flat plate which is thicker than the first light transmissive flat plate, wherein the objective lens of the optical head converges the luminous flux onto the information recording surfaces of the first and the second optical disks, the periphery portion and the center portion of the objective lens being designed so that a reciprocal interference of the respective luminous fluxes which have passed through the periphery portion and the center portion is produced, and the photodetector of the optical head detects the luminous flux, which has been reflected by or transmitted through the information recording surface of the first or the second disk, thereby outputting an electric signal.
 5. An optical head comprising: a light source for emitting a luminous flux; an objective lens for converging the luminous flux onto either the information recording surface of a first light transmissive flat plate functioning as a recording medium or the information recording surface of a second light transmissive flat plate also functioning as a recording medium; separation means for separating the luminous flux, which has been reflected by or transmitted through either the information recording surface of the first light transmissive flat plate or the information recording surface of the second light transmissive flat plate, into a first luminous flux which has been transmitted through the periphery portion of the objective lens and a second luminous flux which has been transmitted through the center portion of the objective lens; and a photodetector for detecting the first and the second luminous fluxes, thereby outputting respective electric signals, wherein the periphery portion and the center portion of the objective lens are designed so that a reciprocal interference of the respective luminous fluxes which have passed through the periphery portion and the center portion is produced.
 6. An optical head comprising: a first light source for emitting a first luminous flux; a second light source for emitting a second luminous flux having a wavelength different from a wavelength of the first luminous flux emitted by the first light source; an objective lens for converging the first luminous flux onto the information recording surface of a first light transmissive flat plate functioning as a recording medium and for converging the second luminous flux onto the information recording surface of a second light transmissive flat plate also functioning as a recording medium; and a photodetector for detecting the luminous flux, which has been reflected by or transmitted through the information recording surface of the first or the second light transmissive flat plate, thereby outputting an electric signal, wherein the objective lens includes a center portion and periphery portion surrounding the center portion, an aberration of the periphery portion is corrected such that a light spot is formed by the convergence of the first luminous flux which has been transmitted through the periphery portion and then transmitted through the first light transmissive flat plate, an aberration of the center portion is corrected such that a light spot is formed by the convergence of the second luminous flux which has been transmitted through the center portion and then transmitted through the second light transmissive flat plate which is thicker than the first light transmissive flat plate, and the periphery portion and the center portion of the objective lens are designed so that a reciprocal interference of the respective luminous fluxes which have passed through the periphery portion and the center portion is produced.
 7. An optical head according to claim 6, wherein the first and the second light transmissive flat plates have a thickness t1 and a thickness t2, respectively, the aberration of the periphery portion is corrected such that a light spot is formed by the convergence of the first luminous flux which has been transmitted through the periphery portion of the objective lens and then transmitted through the first light transmissive flat plate having the thickness t1, and the aberration of the center portion is corrected such that a light spot is formed by the convergence of the second luminous flux which has been transmitted through the center portion of the objective lens and then transmitted through a light transmissive flat plate having a thickness in the range from about (t2×0.7) to t2.
 8. An optical head according to claim 7, wherein the thickness t2 of the second light transmissive flat plate is set at a thickness approximately twice as large as the thickness t1 of the first light transmissive flat plate.
 9. An optical head according to claim 6, wherein, assuming the thickness t1 of the first light transmissive flat plate to be about 0.6 mm and the thickness t2 of the second light transmissive flat plate to be about 1.2 mm, the aberration of the periphery portion is corrected such that a light spot is formed by the convergence of the first luminous flux which has been transmitted through the periphery portion of the objective lens and then transmitted through the first light transmissive flat plate having the thickness of about 0.6 mm, and the aberration of the center portion is corrected such that a light spot is formed by the convergence of the second luminous flux which has been transmitted through the center portion of the objective lens and then transmitted through a light transmissive flat plate having a thickness in the range from about 0.84 mm to about 1.2 mm.
 10. An optical head according to claim 6, further comprising aperture control means for limiting the aperture of the objective lens, wherein the aperture of the objective lens is limited by the aperture control means when the second luminous flux emitted from the second light source is converged onto the information recording surface of the second light transmissive flat plate.
 11. An objective lens according to claim 10, wherein the aperture control means is a wavelength filter for transmitting the second luminous flux emitted from the second light source and blocking the first luminous flux emitted from the first light source.
 12. An optical head according to claim 6, further comprising separation means for separating the luminous flux, which has been reflected by or transmitted through either the information recording surface of the first light transmissive flat plate or the information recording surface of the second light transmissive flat plate, into the first luminous flux which has been transmitted through at least the periphery portion of the objective lens and the second luminous flux which has been transmitted through the center portion of the objective lens, wherein the photodetector detects the first and the second luminous fluxes, thereby outputting respective electric signals, and wherein the electric signal of the photodetector corresponding to the first luminous flux is selected when the first luminous flux is converged onto the information recording surface of the first light transmissive flat plate, and wherein the electric signal of the photodetector corresponding to the second luminous flux is selected when the second luminous flux is converged onto the information recording surface of the second light transmissive flat plate.
 13. An optical head according to claim 12, wherein the separation means is a polarizing hologram.
 14. An optical head according to claim 6, wherein, assuming the wavelength of the first luminous flux emitted from the first light source is in the range from about 600 nm to about 700 nm and the wavelength of the second luminous flux emitted from the second light source is in the range from about 750 nm to about 860 nm, the numerical aperture of the center portion of the objective lens is set to be substantially equal to about 0.45 and the numerical aperture of the periphery portion of the objective lens is set to be substantially equal to about 0.6. 